In a switching power supply apparatus of a current mode control type, a pulse width of a pulse-width-modulation (PWM) signal for attaining a desired output voltage is determined as (Vout/Vin)×Tsw, in which Vout, Vin and Tsw are assumed to be an output voltage, an input voltage and a switching period Tsw, respectively. In this case, the switching power supply apparatus is a voltage step-down type and the pulse width is a pulse width of an ON pulse, which turns on a switching element provided at a high-potential side.
A minimum pulse width cannot be decreased infinitely and is limited because of delay time periods of a pre-driver circuit, a driver circuit, a noise-masking processing circuit and a current detection circuit. In case that the minimum pulse width is longer than a pulse width required to provide a desired output voltage, the desired output voltage cannot be provided and the output voltage rises above the desired voltage value.
To counter this problem, it is proposed to thin out the PWM pulse for suppressing a rise of the output voltage by decreasing a frequency of the PWM pulse, when the output voltage rises to be higher than a predetermined voltage value (for example, JP H06-303766, US 2002-0017897A1).
According to the proposal described above, a switching frequency varies because there arises a time period of thinning out, that is, no generation of the PWM signal. As a result, a frequency band of radio noise generation varies and EMC property changes.